1. Heat the oven to 350°F. Heat the oil in a large deep oven-proof skillet over medium-high heat until it shimmers. Salt and pepper the ribs and cook them, in batches, until they are nicely browned on all sides, about 20 minutes. 2. Remove the ribs and add the onion, carrot, celery, garlic, 2 sprigs of the thyme, and salt and pepper to the skillet. Cook, stirring occasionally, until the vegetables begin to soften, about 5 minutes. Continue cooking until the vegetables are tender and browned, 5 to 10 minutes more. 3. Return the ribs to the skillet. Add the vinegar and enough stock to come up the sides but not over the ribs. Bring the braising liquid to a simmer. Add the tarragon and the remaining thyme, then transfer the skillet to the oven and cook at a very gentle simmer (just an occasional bubble) for 1 hour. Turn the ribs and continue cooking until the meat is tender and comes easily away from the bone, about 1 1/2 hours more. 4. Transfer the ribs and vegetables to a plate. Bring the braising liquid to a simmer and skim off the fat. Reduce the liquid slightly (just so it has a little body), then return the ribs and vegetables to the skillet. Simmer just long enough to reheat the ribs, then serve.